The present invention relates to gamma cameras and more particularly to a method and apparatus for identifying gamma camera photo multiplier tube gain errors and compensating for those errors.
Single photon emission computed tomography (SPECT) examinations are carried out by injecting a dilution marker comprising a compound labeled with a radiopharmaceutical into the body of a patient to be examined. A radiopharmaceutical is a substance that emits photons within a known range of energy levels about a principal energy level. For the purposes of this explanation the principal energy level will be referred to herein as level Z. By choosing a compound that will accumulate in an organ to be imaged, compound concentration, and hence radiopharmaceutical concentration, can be substantially limited to an organ of interest.
While moving through a patient's blood stream the marker, including the radiopharmaceutical, becomes concentrated in the organ to be imaged. By detecting the number of photons having energies that approximate primary energy level Z (e.g. are within plus or minus 10% of level Z) which the organ emits, organ characteristics, including irregularities, can be identified.
To identify photons having energy levels at approximate level Z, a planar gamma camera is used. A gamma camera consists of a collimator, a scintillation crystal and a plurality of photomultiplier tubes (PMTs). A stand supports the detector in a single position with respect to a patient. The collimator typically includes a lead block with tiny holes therethrough which define preferred photon paths. The preferred paths are usually unidirectional and perpendicular to the length of the collimator. The collimator blocks emissions toward the crystal along non-preferred paths.
The scintillation crystal is positioned adjacent the collimator on a side opposite the patient and within a prescribed field of view. The crystal absorbs photons that pass through the collimator on a front surface and emits light from a back surface each time a photon is absorbed. The amount of light emitted from the back surface is proportional to the impacting photon's energy level. For the purposes of this explanation, a photon absorbed by a crystal and emitting light will generally be referred to as an event or a light emitting event and the point of photon impact on the crystal will be referred to as an impact point.
The PMTs are positioned adjacent the crystal and on a side of the crystal opposite the collimator. The portion of each PMT facing the crystal includes a surface area A and will be referred to herein as the PMT face. Light emitted by the crystal passes through the PMT faces and is detected by the PMTs which generate analog intensity signals. When a single photon is absorbed by the crystal, the emitted light is typically absorbed by several different PMTs such that several PMTs generate intensity signals simultaneously, each intensity signal proportional to the amount of light detected and to an internal PMT gain factor Gi associated with the generating PMT. Preferably, all internal gain factors Gi are essentially identical.
A processor receives the intensity signals and deciphers the signals to generate data which can be used by the processor to form an emission image corresponding to the specific camera position. During deciphering, intensity signals corresponding to each event are deciphered in an effort to precisely determine where within the surface area A of a single PMT the event occurred. In addition, intensity signals corresponding to each event are combined to identify a photon energy corresponding to the specific event. Only events having energies within a specific range (i.e. the range of the radiopharmaceutical) are used for imaging. Once all locations of individual events within the desired energy range have been identified, the processor can use the precise locations to create an image of the organ of interest.
Consistent camera operation over time is an important imaging system criterion. To this end, if a specific photon pattern which is directed at a gamma camera generates a specific image during a first imaging period, during any subsequent imaging period when the specific photon pattern is again directed at the same camera, the camera should generate the same image.
Many gamma camera systems require, and the laws in many jurisdictions even dictate, that quality control processes be performed to generally track camera operation. One essentially standard quality control process which is often performed on a daily basis comprises using either an extrinsic or intrinsic photon source to direct an essentially uniform flux of photons toward a camera's scintillation crystal during a quality control test period. The essentially uniform flux of photons is often referred to as a "flood" and the method described herein will be referred to hereinafter as the "flood method". During the flood method, events having photon energies within a window of interest are identified and form an imaging set. Typically the window of interest will be a 20% on-peak acquisition meaning that only events having energy levels within .+-.10% of a primary energy level Z (e.g. 122 keV for 57-CO) are included in the set.
A processor uses the imaging set generated by the 20% on-peak acquisition to generate an image, performs a quantitative evaluation of the image and provides general trend information about image degradation. The image can also be provided to a technician for viewing. Thus, this process can be used to generally identify inconsistent camera operation.
Unreliable PMTs are a primary cause of inconsistent camera operation. More specifically, internal PMT gain factors Gi which vary over time result in inconsistent camera operation. Thus, over time, each PMT within a camera array may become "high", meaning that the PMT's internal gain factor Gi has increased or, in the alternative, may become "low", meaning that the PMT's internal gain factor Gi has decreased. A change in gain factor Gi is often referred to as "drift". Drift can result in image irregularities and therefore must be minimized.
Unfortunately, while the flood method can be used to identify general trends in image degradation, the flood method is relatively ineffective for identifying lesser, albeit still image degrading, amounts of drift. For example, assume that one PMT within an array has drifted and its gain is 5% greater than when it was originally installed in the array. The image generated using the flood method treats all photons having energy levels within the 20% on-peak acquisition range identically. In this case the 20% on-peak acquisition image will indicate very little error as essentially all of the intensity signal will remain within the 20% on-peak acquisition range. This is because most signal energy is at approximately primary level Z the energy spectrum about level Z being shaped somewhat like a bell curve. Thus, a PMT having a gain which is 5% greater than its initial value might only show a 0.2% change in the flood image which would be difficult to identify.
One solution to the drift problem is to identify drift and compensate therefor electronically. Several drift correction methods identify drift by relying on either a built-in reference light source or an external light source. According to these methods, new gamma cameras are subjected to either an external or an internal light source. The light source is used to direct a known photon pattern toward the scintillation crystal and the camera generates a first imaging set related thereto. The first set is stored. The camera is then used during conventional imaging procedures.
Periodically, during quality control procedures, the source is again used to direct the known photon pattern toward the crystal and the camera generates another imaging set. The second set is provided to a processor and compared to the first set to determine if any of the internal gains Gi have drifted.
To correct for drift, an adjustable external PMT gain module is provided for each PMT. As the name implies, each gain module can be used to increase or decrease an external gain factor Ge to compensate for increases or decreases in the internal gain factor Gi. For example, assuming a linear relationship between drift and an external gain factor Ge, where the internal gain factor Gi has decreased by 5%, an external gain factor Ge associated therewith can be increased by 5% to compensate. These gain modules are typically provided in software accessible only by trained technicians and not accessible by imaging personnel. Thus, recalibration typically can only be achieved by a skilled technician.
While these drift correction methods can minimize the effects of PMT drift, they have several important shortcomings. First, any method which relies on a reference photon source to consistently generate a known photon pattern is only as good as the reference source. With these methods the problem of minimizing the effects of drift becomes the problem of ensuring reference stability. While ensuring reference stability is typically easier than ensuring PMT stability, ensuring reference stability is a relatively expensive task.
Second, because a trained technician is usually required to adjust external gain factors Ge, the process identified above is typically not performed as often as it should be. In these cases, drift can affect image quality and even cause system downtime if drift becomes to great.
For these reasons, it would be advantageous to have an apparatus and method for use with a gamma camera for automatically identifying individual PMT drift without the need for a stabilized reference source. In addition, it would be advantageous to have such an apparatus which could perform drift analysis during a standard quality control procedure so that a separate procedure could be avoided. Moreover, it would be advantageous if the apparatus could automatically adjust PMT gain as a function of identified drift.